The invention relates to a sensor fuel cell, and in particular to a sensor fuel cell located on a semiconductor.
German patent application DE 102 55 736 A1 describes, among other things, a sensor fuel cell that can be activated by a first substance situated in its environment. The sensor fuel cell exhibits, on a substrate, a catalytically active anode, a cathode that exhibits a cathode surface freely exposed to the environment, and a proton-conductive membrane that is situated between the anode and the cathode so as to convey protons through from the anode to the cathode. In this disposition the anode is fabricated from palladium and saturated with hydrogen. If atmospheric oxygen from the environment reaches the cathode, a voltage or a current flow is produced when appropriate circuit elements are present.
A fundamental idea is that one of the two electrodes (i.e., the anode or the cathode) is disposed directly on the substrate and completely covered by the membrane and the other electrode. The covered electrode is a reservoir for one of the substances that is employed as reactant in the fuel cell. If oxygen is stored as reactant in or on the covered electrode, the fuel cell can be employed as a sensor and warning device in case of the occurrence of hydrogen in the environment.
A fundamental idea, however, is to fashion a chip-integrable fuel cell on a silicon substrate in the form of a palladium film as an integrated (n) hydrogen reservoir, in order to furnish a power supply source. It is essential that the covered electrode on the substrate is charged with an adequate quantity of the substance or reactant before the membrane and the second electrode are fashioned thereover in completely overlapping fashion. It is accordingly disadvantageous that such a disposition or sensor fuel cell has only a limited lifetime or only a limited service life. Such a sensor fuel cell is no longer employable and must be replaced once the reactant on the covered electrode has been consumed.
The paper entitled “Suspended-Gate- and Lundström-FET Integrated on a CMOS-Chip,” by Ch. Wilbertz, H. P. Frerichs, I. Freund and M. Lehmann, Sensors & Actuators A 123-124 (2005), 2-6, describes a suspended gate and a Lundström FET on CMOS substrates. In the case of the Lundström FET a hydrogen-sensitive film is situated on the gate electrode; in the suspended gate FET, the hydrogen-sensitive film is situated a well-defined distance above the gate electrode. Hydrogen detection takes place via a change in the work function of the hydrogen-sensitive electrodes, by which a threshold voltage of the transistors is altered. The resolution limits of Lundström sensors are typically in the low ppm range while the resolution limit of the suspended gate FET is in the low percent range.
In such Lundström and SG FET sensors it is disadvantageous that these continuously deliver an output signal, which can underlie a baseline drift. The resolution limit of sensors fabricated on this basis is in the ppm and low percent range. Lundström and SG FET sensors are active systems, so that they must be continuously supplied with energy or partly heated in order to operate them.
There is a need for a sensor fuel cell that enables a broader range of applications and at the same time is relatively simple in structure.